Field of the Invention
The present invention relates to an X-ray generating tube generating an X-ray applicable to, for example, medical equipment and non-destructive testing apparatus, and an X-ray generating apparatus and a radiography system that use the X-ray generating tube.
Description of the Related Art
X-ray generating tubes generate an X-ray by applying high voltage in a vacuum container, thus causing an electron source to emit an electron beam and creating a collision between electrons and a target made of a metal material that has a high atomic number, such as tungsten.
The voltage applied between a cathode, which includes the electron source, and an anode, which includes the target, is generally about 10 kV to about 150 kV, although varying depending on the use of the generated X-ray. The trunk of the vacuum container is built from an insulating tube that is made of an insulating material such as glass or a ceramic material in order to keep the interior under vacuum and to electrically insulate the cathode and the anode from each other.
When an X-ray generating tube is driven to cause the electron source to emit electrons, scattered electrons and secondary electrons are generated in the X-ray generating tube and are in some cases captured on the inner surface of the insulating tube to charge the inner surface. With the inner surface of the insulating tube charged, an electric field thereof disrupts the trajectory of an electron beam to change the irradiation points and focal spot size of the electron beam, and consequently changes the focal position and dose of an emitted X-ray. In addition, the location and amount of charge on the inner surface of the insulating tube vary depending on the distribution of points irradiated with the scattered electrons and secondary electrons, and the resultant difference in electric potential on the inner surface of the insulating tube may lead to a discharge that damages the insulating tube.
There is disclosed in Japanese Patent Application Laid-Open No. S58-44662 a technology of preventing the accumulation of electric charges by forming a conductive film that is made from fine metal particle groups and a glaze along the inner circumference of the insulating tube.
However, no particular consideration is given to the connection between a low conductivity film and the electrodes in Japanese Patent Application Laid-Open No. S58-44662. Accordingly, poor connection between the low conductivity film and the electrodes hinders the scattered electrons and the secondary electrons from freeing, thereby putting the conductive film itself into a charged state. The charged conductive film may disrupt the electron beam trajectory and change the X-ray output.